thing is, it really depends on the type of topic.  For example, threads asking for suggestions in the project topics should be held open for longer.

I work on about a year these days. For dormant threads.
I would like to highlight that I lock old threads, based on the last post date, not the thread start date.

3/6 months can be a bit too long and too short depending on the threads section/context. Safest bet is to use a year (plus its far easier for me).

I haven't yet had any suggestions/replies for specific forums to have longer/shorter dormant thread locking policies, except for the Programming Forum (Hooman - never lock them, as they get added to after extended periods of time) & The Main Projects (Under control of sub-forum Mods - So they can control idea flows)

Edit: See here for the others: https://forum.outpost2.net/index.php?topic=4230.0

QuoteI'm glad this thread didn't get locked.  I think that there should always be at least one thread open in every forum/subforum.

I can't remember where, and nor can I be bothered to search for the post/thread.  But it has been generally accepted that if you want to continue to comment on an old locked thread you start a new one and reference, with a link, the old one.  If a Mod deems the new thread to be a continuation of the old one it can be merged.  The majority of the time the new thread is at a tangent to the old one and therefore not merged.

Edit -  Added reference to my Spring Cleaning post, couple of grammatical/spelling errors I noticed